Silicon fixture supporting silicon wafers during high temperature processing

ABSTRACT

A process for hydrogen annealing silicon wafers that have been cut from an ingot and polished on both sides, thereby removing crystal originated pits (COPs) in their surface. The wafers are then stacked in a tower having at least support surfaces made from virgin polysilicon, that is, polysilicon form by chemical vapor deposition, preferably from monosilane. The tower may include four virgin polysilicon legs have support teeth slotted at inclined angles along the legs and fixed at their opposed ends to bases. The wafers so supported on the virgin polysilicon towers are annealed in a hydrogen ambient at 1250° C. for 12 hours.

RELATED APPLICATION

This application is a continuation of Ser. No. 09/792,989, filed Feb.26, 2001 and now issued as U.S. Pat. 6,727,191.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to semiconductor processing. Inparticular, the invention relates to the method and apparatus forremoving defects from a silicon wafer prior to device fabrication.

2. Background Art

The increased density of devices formed on advanced silicon integratedcircuits has required raw silicon wafers to have a further reduceddensity of defects. Although silicon wafers are substantiallymonocrystalline, they may suffer from several types of surface and bulkdefects.

A slip defect occurs when the silicon is not perfectly monocrystalline.Instead, boundaries may develop in the bulk silicon across which thesilicon atoms do not perfectly line up. Slip is believed to arise fromshear stress. If the slip is large, a silicon plane on one side of theboundary may gradually transition between multiple silicon planes on theother side. Such a large slip renders that part of the silicon waferessentially useless for integrated circuits. There are various degree ofslip, but nearly invariably slip propagates and worsens with additionalthermal processing. Strain introduced by slip can cause substantialdifficulties, such as accelerated dopant diffusion in the vicinity ofthe strained material, resulting in a non-uniform diffusion density, orconcentration of impurities around the slip.

We believe that many occurrences of slip arise during high temperatureprocessing steps in which the silicon wafer moves over a supportingsurface of a wafer fixture, such as a boat or tower, primarily becauseof differential thermal expansion of the two materials. Slip seems tobecome a particular problem when minimum feature sizes are less than0.18 μm, which is the approximate size for advanced processing at thecurrent time.

Another type of defect produces what are called crystal originated pits(COPs), which are agglomerated vacancy defects occurring at the wafersurface. Crystalline silicon typically contains a significant number ofatomic defects in the form of vacancies and interstitials. The vacanciestend to agglomerate into larger voids distributed throughout thesilicon. Any such void exposed at the wafer surface appears as a pit.These voids and pits are generally formed in what is otherwise auniform, monocrystalline wafer. COPs become a major problem at minimumfeature sizes of 0.13 μm, the size being contemplated for the nextgeneration of integrated circuits.

There are several conventional methods of reducing if not eliminatingthe occurrence of crystal originated pits. In one method, the wafer iscut from a Czochralski-grown or float-zone crystallized ingot, isrounded, and has flats or other orienting indicia cut into itsperiphery. For some applications such as solar cells, the silicon wafermay be polycrystalline. The wafer is often then polished on both sidesalthough in the past polishing has been limited to one wide. At thisstage, a large number of COPs are typically present. In one method toremove the COPs, the wafer is then annealed at high temperature, forexample, 12 hours at 1250° C., in a hydrogen environment. The hightemperature promotes diffusion of silicon interstitials into the pits,thus planarizing the surface and eliminating the pits. The hydrogen isbeneficial in promoting the diffusion of interstitials throughout thesilicon wafer. The mobilized interstitials fill the surface pits.

The high-temperature COP anneal has not however been completelysuccessful. One problem is that the annealing temperature is not thatfar below the melting point of silicon, which is approximately 1414° C.The long, hot COP anneal is likely to cause the silicon wafer supportedat a minimum number of points to sag. Even if the wafer returns to itsplanar shape upon cooling, stress is likely to be thereby introduced.Sag becomes even more of a problem as the transition progresses from 200mm to 300 mm wafers.

Quartz boats and towers are inadequate for these high temperaturesbecause they sag at these temperatures. As a result, silicon carbideboats or towers are typically used for high-temperature processingbecause silicon carbide remains strong at significantly highertemperatures. However, silicon carbide presents its own set of problems.It is likely to contain a significant concentration of heavy metals,which are also mobilized by the high-temperature hydrogen anneal andwhich are very deleterious to semiconductor devices into which the heavymetals may diffuse. Solutions are available to address these problems,but they are both expensive and not completely effective.

It is further been observed that a COP anneal or other high-temperatureanneal using a silicon carbide tower, even of the best design, is likelyto introduce slip at the locations at which the tower supports thewafer. We believe the slip arises due to the differential coefficientsof thermal expansion between silicon and silicon carbide and due to thesilicon carbide being harder than silicon.

For these reasons, it is desired to provide a method and apparatus formore effectively performing a high temperature anneal of a siliconwafer.

SUMMARY OF THE INVENTION

The invention includes an annealing method for removing defects from astock silicon wafer after it has been cut from an ingot and preferablypolished on both sides. A plurality of wafers are stacked in a supportfixture, for example, a vertically extending tower, having at leastsupport surfaces composed of silicon, preferably virgin polysilicon.

More preferably, the tower is composed of three or four silicon legs,preferably composed of virgin polysilicon, joined to two end bases.Slots are cut into the four legs to form inclined teeth having supportsurfaces disposed at locations corresponding to about 0.707 of the waferradius. These locations equalize the mass of the wafer inside andoutside of the support points. The support points for four legs arepreferably arranged in a rectangular pattern centered, most preferably asquare pattern, centered about the wafer center, thereby reducing wafersag. The support points for three legs are preferably arranged in apattern of an equilateral triangle centered on the wafer center.

The wafers supported in the silicon tower are subjected to ahigh-temperature anneal in a hydrogen ambient, for example, at 1250° C.for 12 hours.

The tower may also be advantageously used for other processes involvinghigh temperatures above 1100° C. and may also be used forlower-temperature processes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an orthographic view of a silicon tower usable in the processof the invention.

FIG. 2 is an exploded orthographic view of one of the legs of the towerof FIG. 1.

FIG. 3 is an exploded elevational view of the teeth and stem portion ofone of the legs of the tower of FIG. 1.

FIG. 4 is an axial sectioned view of the tower of FIG. 1 illustratingthe arrangement of teeth and support areas.

FIG. 5 is a simplified schematic representation of an annealing oven inwhich the method of the invention may be practiced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

We believe that silicon carbide, even covered with a layer of CVDsilicon carbide, is inappropriate for use as support towers inhigh-temperature processing of silicon wafers. Silicon carbide is a hardmaterial, and a significant differential coefficient of thermalexpansion (CTE) exists between silicon and silicon carbide so that thesilicon wafers and silicon carbide tower move relatively to each otheras they are cycled up to the annealing temperature and back down to roomtemperature. During the temperature cycle, the differential thermalexpansion causes the silicon wafer to drag over the harder siliconcarbide. So far, perfectly smooth silicon carbide has not beenavailable, and in any case there would still be dragging at some level.Furthermore, we believe that metals will inevitably diffuse from thesilicon carbide into the silicon, particularly over the long periods ofcommercial production desired for expensive towers.

According to one aspect of the invention, a tower or boat used tosupport wafers in a high-temperature anneal of stock silicon wafers hasat least support surfaces formed of silicon, preferably polysilicon, andmore preferably virgin polysilicon. Boyle et al. have disclosed thefabrication of such a tower in U.S. patent application, Ser. No.09/608,291, filed Jun. 30, 2000, now issued as U.S. Pat. No. 6,455,395,and incorporated herein by reference in its entirety. Virgin polysiliconis polycrystalline silicon formed by the chemical vapor deposition (CVD)of silane and/or chlorosilane. Virgin polysilicon is conventionally usedas the source material for the Czochralski growth of monocrystallinesilicon ingots. Although trichlorosilane (CHCl₃) is the most prevalentlyused CVD precursor for semiconductor applications, virgin polysiliconformed from monosilane (SiH₄) is preferred for towers because of theabsence of trace amounts of chlorine. Virgin polysilicon ofextraordinarily high purity is commercially available.

A form of a tower 10 is illustrated in the orthographic view of FIG. 1.It includes two bases 12, 14 and four legs 16, 18, 20, 22 permanentlyaffixed at opposed ends to the two bases 12, 14. Preferably, at leastthe legs 16, 18, 20, 22 and, more preferably, also the bases 12, 14 aremachined from separate pieces of silicon. The legs 16, 18, 20, 22 aremore preferably machined from virgin polysilicon since they contact andsupport the wafers. The bases 12, 14 need not be fabricated from virginpolysilicon because they do not contact the silicon wafers. All thepieces are then joined together. The machining and joining processes aredescribed in the cited patent application to Boyle et al. Each leg 16,18, 20, 22 has a large number of generally parallel teeth 24, moreclearly shown in the expanded orthographic view of FIG. 2 for the legs18, 20 with short teeth 24 b, which are machined into the legs to formslots 26 between the teeth 24. FIG. 1 has been simplified. In oneproduct, 118 sets of teeth are spaced along about 1 m of a tower tosupport a large number of wafers 30 in a horizontal, parallelarrangement.

As more clearly shown in the detailed elevational view of FIG. 3, theteeth 24 a, 24 b are cut to incline upwardly at a set angle of between87° and 89½° from the axis of the legs, but level support areas 28 a, 28b are machined into the end of the teeth 24 a, 24 b to support a wafer30 at a total of four points far removed from the leg stems 32 a, 32 b.

For presently developed silicon integrated circuit technology, it doesnot appear necessary to polish the support areas 28, 28 b. However, itis anticipated that as the circuit technology advances, polishing may berequired. There are two types of polishing contemplated. Chemicalmechanical polishing (CMP) uses a silica slurry in an alkaline colloidalsuspension and produces a very smooth, damage-free surface. Diamondpolishing with small (1 to 31 μm) abrasive particles can produce anequally smooth surface but with substantial sub-surface work damage.

As shown in both the orthographic view of FIG. 1 and the axiallysectioned view of FIG. 4, two legs 14, 16 having long teeth 24 aprojecting from their leg stems 32 a are located on the side of thetower while two legs 18, 20 having shorter teeth 24 b are located on theother side. The shorter teeth 24 b project generally radially inwardlyfrom their leg stems 32 b with respect to the center of the tower 10while the longer teeth 24 a project towards the entry side of the towerto allow insertion of the wafers. The distance between the leg stems 32a of the longer-tooth legs 16, 22 is slightly larger than the diameterof the wafer 30 being inserted into the tower. This geometry allows allsupport surfaces 28 a, 28 b to form a square pattern at locationscorresponding to about 0.707 (2^(−1/2))of the wafer radius. Arectangular pattern and more advantageously a square pattern reduce themaximum sag for points of the wafer far removed from the supportsurfaces. The radial position corresponds to a radius having an equalweight of wafer inside and out, thereby minimizing thermally induced sagand sag deformation and stress. A variation of 5% about this radiusshould introduce no significant problems, but in fact the support areas28 a, 28 b are large enough to easily encompass the 0.707 position.Expansion slots 34 cut into the bases 14, 16 along the wafer insertiondirection connected to a center relief circle 36 relieve any thermalstresses built up in the bases.

An alternative tower design uses three legs with the support areasarranged in the pattern of a triangle, preferably an equilateraltriangle, symmetrically arranged about the wafer center and preferablydisposed at the 0.707 positions. The three-point support increases chipyield since almost invariably the supported area of the wafer does notproduce a working chip. However, the triangular support pattern producesa larger maximum sag. Hence, the choice of three or four legs is basedat least partially on the maximum processing temperature.

Such towers are capable of supporting wafers for extended periods in ahigh temperature anneal. The entire tower is formed of silicon sodifferential thermal expansion between it and the wafers is minimized.The silicon material, particularly at the supporting areas, hassubstantially the same hardness as the silicon wafers being supported.Accordingly, even if there is some dragging of the wafer on the towersurfaces, the dragging is unlikely to induce defects in the wafer. Theplacement of the teeth support surfaces minimizes sag and strain in thewafer, thereby reducing if not eliminating slip in the crystallinesilicon wafer material. Because at least the legs may be formed ofvirgin polysilicon of very high purity, particularly in regards to heavymetals, impurity diffusion from the tower to the wafers is substantiallyeliminated.

Such a tower is consistent with the cited hydrogen anneal at 1250° C.for 12 hours. Wafers are loaded into the above described silicon tower10. The loaded tower 10 is placed in a conventional annealing oven 40,schematically illustrated in FIG. 5, which is controllably heated byresistive heaters 42 to the desired annealing temperature. Hydrogen andargon gas are flowed into the annealing oven 40 from gas supplies 44, 46to supply the 25% H₂ ambient that is substantially otherwise inert. Theanneal is continued for the desired time of, for example, 12 hours.

Experiments have shown that such an anneal all but eliminates thecrystal originated pits (COPs) without introducing any detectable slipinto the wafers. Of course, the invention is not limited to thisparticular annealing schedule. Somewhat higher temperatures, of coursebelow the melting point of silicon of 1414° C., will shorten therequired annealing time. An annealing time of at least an hour isbelieved necessary to produce the desired interstitial diffusion belowannealing temperatures that would soften the silicon. Temperatures aslow as 1100° C. will produce a desired effect, but with the requirementof longer annealing times. Further, it is apparent that such ahigh-temperature tower can be used for processes involving lowertemperatures. Indeed, use of lesser quality quartz or silicon carbidetowers for medium-temperature processing after the high-temperaturehydrogen anneal with a silicon tower of the invention is likely tocompromise the purity and slip-free crystalline state of the wafer.

Other hydrogen-containing processing gases may be used with thehigh-temperature hydrogen anneal of the invention. Forming gas is acommercially available gas that is considered relatively non-explosive.It contains 5 to 7% of H₂ by volume, the remainder being N₂, which inmany applications is considered inert. Thus, a minimum of 5% H₂ in anotherwise inert carrier will provide a beneficial high-temperatureanneal.

Although a vertically oriented tower has been described, the sameeffects can be achieved with a horizontally oriented boat, assuming theannealing oven is configured for a boat. The invention is not limited tothe processing of silicon wafers. Other substrate materials, forexample, silica optical boards, require high-temperature processing thatwould benefit from the above described silicon support fixture.

The invention becomes increasingly more important with the advent of thelarger 300 mm wafers and a further reduction in minimum feature sizesbelow the typical sizes of COPs and slip. However, the invention may aswell be applied to the smaller wafer most prevalent at the present time.

1. A high-temperature treatment process, comprising the steps of:supporting first sides of a plurality of silicon substrates on a supportfixture having support surfaces consisting essentially of silicon;disposing said support fixture supporting said silicon substrates in anoven; flowing an ambient gas comprising hydrogen into said oven; andtreating said silicon substrates in said ambient gas at a temperature ofat least 1100° C.
 2. The process of claim 1, wherein said supportfixture comprises a plurality of legs extending along respective axesand composed of virgin polysilicon and including teeth projecting atinclined angles from stem portions of said legs, said support surfacesbeing formed at ends of said teeth to extend perpendicularly to saidaxes.
 3. The process of claim 2, wherein said inclined angles are in arange of 87 ° to 89½° with respect to said axes.
 4. The process of claim2, wherein there are four of said legs and said support surfaces aredisposed in a square pattern to support said substrates.
 5. The processof claim 4, wherein said support surfaces are disposed in said squarepattern at locations located at approximately 0.707 of four radii ofsaid substrate.
 6. The process of claim 2, wherein there are three ofsaid legs and said support surfaces are disposed in an equilateraltriangle to support said substrates at locations located atapproximately 0.707 of three radii of said substrate.
 7. The process ofclaim 1, wherein said second sides of said substrates are polished. 8.The process of claim 1, wherein said substrates are substantiallycircular silicon wafers, further comprising the prior steps of: cuttingsaid wafers from one or more silicon ingots; and polishing at least saidsecond sides of said wafers.
 9. A high-temperature wafer processingmethod, comprising the steps of: providing a tower comprising aplurality of silicon legs, extending along respective axes, and eachhaving a plurality of teeth cut therein at inclined angles with respectto said axes and having support areas formed on ends of said teeth toextend perpendicularly to said axes, and two bases fixed to opposed endsof said plurality of legs; supporting a plurality of substrates on saidsupport areas; and processing said substrates at a processingtemperature of at least 1100° C.
 10. The method of claim 9, wherein saidplurality of legs consist of four legs and said support surfaces aredisposed in a square pattern about a wafer center.
 11. The method ofclaim 9, wherein said plurality of legs comprise three legs and saidsupport surfaces are disposed at 0.707 of three wafer radii in anequilateral triangle pattern about a wafer center.
 12. The method ofclaim 9, wherein said processing includes exposing said wafers to atreatment ambient comprising hydrogen at said processing temperature.13. The method of claim 9, wherein said processing temperature is atleast 1250° C. and is maintained for an extended predetermined length oftime.
 14. The method of claim 9, wherein said legs comprise virginpolysilicon.
 15. The method of claim 9, wherein said inclined angles arein a range of between 87° and 89½°.
 16. A wafer supporting tower,comprising: two bases comprising silicon; a plurality of legs joined atopposite ends thereof to said two bases and comprising silicon; and aplurality of slots cut into said legs to form teeth extending from stemportions of said legs at angles of between 87° and 89½° and havingsupport surfaces formed on ends thereof for supporting wafers thereon.17. The tower of claim 16, wherein said legs comprise polysilicon. 18.The tower of claim 17, wherein said polysilicon is virgin polysilicon.19. The tower of claim 18, wherein said virgin polysilicon ischlorine-free virgin polysilicon.
 20. The tower of claim 16, whereinsaid legs consist essentially of silicon.
 21. The tower of claim 1,wherein second sides of said substrates opposite said first sides aresubstantially monocrystalline.
 22. The process of claim 21, wherein saidsupport fixture comprises a plurality of legs extending along respectiveaxes and composed of virgin polysilicon and including teeth projectingat inclined angles from stem portions of said legs, said supportsurfaces being formed at ends of said teeth to extend perpendicularly tosaid axes.